Expandable foam products, which find use as packaging, cushioning, insulating and structural materials, typically consist of a phase of open or closed pores or cells dispersed throughout a polymer matrix. A wide array of processes have been devised for developing the cell phase in these products, including adding a gaseous "blowing agent" to the polymer during processing, producing a gaseous blowing agent by chemical reaction within the polymer during processing, and forming the product from polymer granules to obtain a cellular structure. In one particularly popular process, a gaseous blowing agent is incorporated into a molten thermoplastic material to form a mixture which may then be molded to a desired shape, such as by extrusion. After molding, applied heat or reduced pressure causes the blowing agent to expand, forming a cellular structure within the thermoplastic matrix. The effectiveness of a particular blowing agent will depend largely upon the polymer composition in which it is incorporated, the method of incorporation, the process conditions, the additives used, and the products sought.
Blowing agents work by expanding the polymer to produce a cellular structure having far less density than the polymer itself. In processes in which a blowing agent is incorporated into a molten thermoplastic polymer, bubbles of gas form around "nucleation sites" and are expanded by heat or reduced pressure. A nucleation site is a small particle or a conglomerate of small particles that promotes the formation of a gas bubble in the polymer. Additives may be incorporated into the polymer to promote nucleation for a particular blowing agent and, consequently, a more uniform pore distribution.
Once bubbles of the blowing agent have expanded to form the cellular structure, the structure is maintained by replacing the blowing agent in the cells with air. Diffusivity of the blowing agent out of the cells relative to air coming into the cells impacts the stability of the foam over time and whether the cells of the foam may collapse. Additives may be incorporated into the polymer and process conditions may be adjusted to assist in controlling the diffusivity of the blowing agent, to promote foam stability, and to limit collapse of the foam to acceptable limits.
Many methods are available for adding a blowing agent to a polymer during processing to produce a foam. In one method pertinent to the present invention, the blowing agent is mixed with a molten thermoplastic polymer under pressure, and the mixture is then extruded through a forming die into a zone of reduced pressure. Shaped extruded foams may be produced by this method using a forming die of a desired configuration. Plank, which can be cut to a desired shape, and thin foam sheets may also be produced in this manner.
Prior art processes for forming expanded foam products from thermoplastic polymers typically used halogenated hydrocarbons as blowing agents. The halogenated hydrocarbons include the chlorofluorocarbons ("CFCs") and hydrochlorofluorocarbons ("HCFCs"). CFCs and HCFCs are readily impregnable in thermoplastic polymers and are readily expandable under relatively mild conditions. CFCs and HCFCs generally produce foams of high quality with a minimum of processing difficulty. The pore size is controllable, the foam has good stability with minimum tendency to collapse after a period of time, and the surface characteristics of the foam are smooth and desirable. Also, CFCs, HCFCs and other halogenated hydrocarbons typically are either not flammable or are of low flammability, which greatly reduces the care with which they may be used. These compounds have the further advantage of low toxicity. However, CFCs, HCFCs and other halogenated hydrocarbons have been linked to ozone depletion in the atmosphere. As a result of concern over the ozone layer, the use of these materials is being phased out in favor of materials which are more friendly to the ozone layer, such as hydrocarbons.
Although hydrocarbons are readily available, inexpensive and very compatible with polyethylene and other polymer matrix materials, thereby permitting wide processing variability, they present their own unique problems. Foremost among these problems is the greater flammability of these materials. Other problems with hydrocarbon blowing agents may include toxicity or environmental incompatibility. Moreover, the hydrocarbon blowing agents are slow to permeate through the expanded foam structure, such that the flammability and other problems associated with these materials persist in the foam structures for longer periods of time. Safety concerns have therefore mandated that manufacturers of these products store them for excessively long periods of time to enable the blowing agents therein to dissipate to levels below their lowest explosive limit so that the products are safe enough to be shipped to and used by customers.
The problems associated with the use of hydrocarbon blowing agents would be minimized if a majority of the blowing agent could be removed from the expanded foam structure as quickly as possible. Although attempts have been made in the past to do just that, these attempts have proven to be unsatisfactory. Thus, in U.S. Pat. Nos. 5,424,016 and 5,585,058 to Kolosowski, an expanded foam structure is perforated with a multiplicity of channels extending from one surface of the structure to the opposite surface. These channels shorten the path through which the blowing agent must travel to be diffused from the interior of the structure to the atmosphere. However, these channels also decrease the mechanical properties of the foam, including its compression strength, resistance to creep, cushioning ability and the like. U.S. Pat. No. 5,776,390 to Fiddelaers et al. also teaches perforating an extruded foam in order to facilitate the dissipation of the blowing agent. In this method, however, the perforations are made from one side of the foam and extend through only about 60-97 percent of the foam thickness so as to avoid removal of the surface skin from the foam. The problem with this approach, however, is that perforating from a single side while the foam is still hot causes residual stresses to develop in the foam, thus resulting in foam warpage.
Despite the efforts that have been made in the past, there remains a need for production methods which will accelerate the removal of a majority of the blowing agent from the expanded foam structure without detrimentally affecting its resulting strength, cushioning properties or overall foam quality.